This relates a magnetic compass arrangement for use in a machine and, more particularly, to a method of using a magnetic compass arrangement in a movable machine or vehicle, such as for example an excavator. Sensor arrangements which sense the direction of the magnetic field of the earth and provide outputs for display and use by a control system have been used for some time. Such arrangements have been subjected to, and have had to compensate for, “hard iron” effects and “soft iron” effects. Both of these effects distort the shape of the earth's flux lines in the local area of the sensor, but do so in different ways. Hard iron effects arise from permanent magnets and magnetized iron or steel in the vehicle. Hard iron effects cause the earth's magnetic field to be offset in a particular direction and do not change as the machine, such as an excavator, is rotated about a vertical axis. The errors induced by hard iron effects go through a single cycle during a complete rotation of the machine. Soft iron effects arise from the interaction of the earth's magnetic field with the magnetically permeable material of the excavator. Soft iron effects change as the machine is rotated, and the errors induced by soft iron effects go through two cycles during a complete rotation of the machine.
Typically, the earth's magnetic field is sensed with three magnetic field sensors which are oriented to detect the strength of the earth's field in three orthogonal directions, the X, Y, and Z directions of the sensor's coordinate frame of reference. From the relative outputs of the three sensors it is a simple matter to determine the relative orientation and strength of the field with respect to the sensor. It will be appreciated that if the sensor coordinate reference frame is not aligned with the local level plane coordinate reference frame of the worksite, then the sensed indications of the magnetic field from the X, Y and Z coordinate reference frame of the sensor must be transformed to the local level plane coordinate reference frame to determine the direction of the flux and therefore the heading of the machine with respect to the worksite.
Various approaches have been used in the past to compensate magnetic compass systems for hard iron effects and soft iron effects. Many of these prior art calibration techniques have required extensive measurement of magnetic field strength with the machine and compass oriented in various directions, as a part of an extended cumbersome calibration routine.